The present invention relates to a medical diagnostic method and apparatus for x-ray image presentation on a video monitor and, more particularly, to a method and apparatus for reducing flicker of an x-ray image on a video monitor when the cine frame rate is different from the video field presentation rate on the video monitor.
In a typical imaging system used for medical diagnostics, primary imaging radiation, such as x-ray radiation which has been intensity modulated by passage through a patient, strikes the input screen of an image intensifier tube where it is converted to an electron latent image. Electrodes contained in the tube minify the image and accelerate the electrons toward a luminescent output screen of the image intensifier tube. An image having increased brightness is produced on the output screen in accordance with the spatial modulation of the x-ray radiation. A television camera and monitor are used to display the image. Frequently, a photographic camera or cine camera is also used to record images of diagnostic interest.
High image quality, as measured by image resolution, contrast, and x-ray photon noise, is very desirable in such applications of the imaging apparatus. However, when the x-ray exposures during cine procedures occur at a rate different from the video field presentation rate of the video monitor, a phenomenon referred to as "flicker" occurs. For example, the typical video system presents to the viewer 60 interlaced images (fields) per second in order to provide a flicker-free display. Cine images may be acquired at 15 to 90 frames per second and these are frequently viewed, and possibly recorded, simultaneously through the video system. In each of these arrangements, the x-ray source which is creating the images to be viewed by exposing a patient to x-ray radiation, is pulsed synchronously with the cine recording rate. Since the video field rate is different than the cine recording rate, the video image appears to flicker. At a recording rate of 60 frames per second, the video image does not flicker since the field presentation on the video monitor occurs at the same rate as image acquisition and exposure repetition rate by the x-ray source.
The flicker effect is most pronounced when using a low lag pick up tube, i.e., one which has a substantially reduced image intensity after a first scan of the image on the camera target. This type of tube is preferred in many applications such as those used in viewing moving structures such as blood vessels since the presentation has the least amount of blur for moving structures. The degree of flicker can be reduced by using a pick up tube with a greater lag so that there is a substantial residual signal to read during a time when an un-illuminated field of the camera target is being read. However, the tube with the greater lag results in objectional blurring.
Before continuing with the description, it is important to note that the flicker effect is created by a difference in intensity between video fields and not from the particular display frame rate of the video monitor. As is well known, the properties of the human eye are such that if the frequency of frame projection on a video monitor is above 15 frames per second, the motion will appear to be continuous. This occurs because the eye retains the image which it receives for a fraction of a second after the image is removed. Therefore, if many views are presented in succession, the persistence of vision will integrate them and give the impression of seeing continuous motion. Frame rates for conventional motion picture projection are usually 24 pictures per second. Even though a frequency of 24 frames per second is rapid enough to allow for continuity of motion, it is not rapid enough to eliminate the flicker effect. This results because the human eye, although it will integrate motion at 24 frames per second, will not totally integrate brightness. .In order to overcome the flicker effect, it is necessary to present a sequence of pictures to the eye at equal intensity at a rate faster than 24 frames per second. In motion picture- practice, the solution to the flicker effect caused by presentation at 24 frames per second is overcome by showing each frame of a projected image twice before advancing to a next frame. In this manner, the eye sees a frame rate of 48 frames per second. In a television video monitor, the flicker effect is avoided by providing 30 frames per second but by interlacing two fields so as to actually appear to present 60 different images or fields per second. In essence, the television system produces half of a picture in one field of view and the remaining half in a second field. In this manner, the eye is able to integrate the brightness over the interlaced frames and the flicker effect is avoided so long as the brightness of each field interlaced to form a frame is the same.
Based upon the above discussion, it will be apparent that if the exposure repetition rate in the x-ray system is such that the images presented on the television monitor are not of equal brightness, the eye will perceive the difference in brightness as a flicker effect. In a common exposure repetition rate of 30 exposures per second, the camera tube will provide a first image for the first field scan presentation on the video monitor which has a substantially high level of intensity. However, as the camera generates this first video field by scanning the target of the camera tube, the intensity of the image on the tube target is substantially reduced. In most tubes, the residual image on the tube target will be at less than 50% of the original image. Consequently, when the tube target is scanned to generate a second field of view, the intensity of the image presented to the video monitor will be substantially reduced and the eye will detect the reduction as a flicker effect.
There are several different types of camera tubes available each of which has different residual image characteristics as a function of the type of target material used in the camera. One of the solutions to the flicker effect has been to use a camera tube which has a greater lag or residual image so that there is a substantial residual image to read during the scanning of the un-illuminated field. An alternate solution to the problem has been to store the image from the illuminated field in some type of memory and to then display the same image a second time for the second video field. However, image storage devices for performing this function are relatively expensive. In addition, while the image storage device may provide a satisfactory solution at 30 cine frames per second, at 45 frames per second a cine exposure pulse may occur at the beginning of the video field, in the middle or not at all during any one field. If only those fields are displayed for which the exposure took place at the beginning of the field, then the effective video frame rate would be reduced to 20 frames per second and increase the flicker effect.